Phytophthora infestans - Potato late blight

A tiny fungus with a massive impact

The potato’s worst enemy is a fungus with the evocative name Phytophthora infestans, better known as potato late blight.

Potato plant infected with Phytophthora. The leaves and stems are infected first

Infected tuber

Phytophthora is part of the Oomycetes group (water moulds), which used to be classed as fungi, and now tend to be classed as algae.

Photos: Syngenta

Many consider it to be the most devastating plant disease of all, because Phytophthora is so quick and agile that it can cause enormous damage within a very short space of time - especially during the warm damp weather conditions it favours. Moreover, it is so versatile that it has so far survived every strategy used to control it and has responded by evolving newly adapted forms. Even today Phytophthora still causes global yield losses of around 20%.

The initial symptoms of Phytophthora infection appear on the stems and leaves. Patches form at the edges of the leaves, grey-green at first and turning brown later, and spread quickly during damp weather. A white fungal coating forms on the underside of the leaves. The leaves then dry up or rot. Phytophthora spreads via spores, which use a germ tube to penetrate the plant tissue. The fungus spreads mainly on the wind or is washed into the soil by rain, where is also infects the tubers. This results in inedible potatoes with blue-grey patches, whose flesh eventually turns brown and rots.

Phytophthora can overwinter in the tubers and a single infected tuber planted in the spring is all it takes to cause an epidemic in the potato crop.

A fungus makes history

Phytophthora gained notoriety and had a fateful impact on world history during the events in Ireland in the mid-19th century. For several successive years the fungus virtually destroyed the entire potato harvest in the country, triggering mass starvation as a result of which around one million people died and a further 2 million emigrated to Australia and North America. The Kennedy family was amongst the emigrants and decades later John F. Kennedy was elected president of the United States of America.

But how could such a disaster happen? Impoverished Irish farmers lived almost entirely on a staple diet of the nutritious yet humble potato. The cereals they grew had to be handed over to their landlords for export to England. So year after year they grew nothing but potatoes for their own subsistence. Exactly how and when Phytophthora first appeared on Irish soil can only be guessed at. It is possible that it arrived with a shipment of seed potatoes from Mexico in the ill-fated year of 1844 and had an easy time with the few potato varieties grown in Ireland, none of which had any natural resistance to Phytophthora. Perhaps the mysterious disease had been around for longer and it was only the particular circumstances that led to the disaster. It was certainly a particularly cold, wet summer, on top of which the previous year’s harvest was so exceptionally abundant, that it resulted in a glut of potatoes, which the farmers threw onto the fields in the spring. Phytophthora had overwintered and was then able to spread very rapidly via the infected tubers.

Monoculture, steam engine or divine retribution? A hunt for the cause

It was only discovered some time later that this devastating epidemic was caused by a fungus. All kinds of theories were put forward initially. One theory was that monoculture had leached the goodness from the soil. Contemporary critics of industrialisation and the accompanying environmental pollution speculated that it was caused by electrical impulses transmitted by steam locomotives. Others blamed the newly discovered phosphorus matches, whilst still more put it down to unknown gases from outer space. And of course some believed it was divine providence as punishment for wastefulness in the years of plenty. A little while later an English clergyman and amateur naturalist was examining leaves infected with Phytophthora under the microscope and maintained that the plants were tightly bound in countless tiny filaments. He was dismissed as a crank and it was another 15 years before the German plant pathologist Anton de Bary confirmed his theory. He was also able to explain that the fungus spread via tiny spores which are carried from plant to plant.

At that time, after the Irish exodus, Europeans had made their way to South America, the land of the potato, to look for varieties that were resistant to Phytophthora. South America is the homeland of the potato and it is here that the greatest species diversity is found. The plant hunters found “Solanum demissum”, a resistant Mexican wild species, whose genes characterise four-fifths of potatoes today.